Bump to newer 2.26.* Nix version

Needed one more thing before trying out using `LegacySSHStore` directly.

Flake lock file updates:

• Updated input 'nix':
    'github:NixOS/nix/674a87462cb93f605d4fbeef607d3453e7e5a7d8?narHash=sha256-TBoHqnIdVWhsBcL05vO2B1hSl9m//5Mz2NU%2BPMk3h3Y%3D' (2025-02-16)
  → 'github:NixOS/nix/e310c19a1aeb1ce1ed4d41d5ab2d02db596e0918?narHash=sha256-q/RgA4bB7zWai4oPySq9mch7qH14IEeom2P64SXdqHs%3D' (2025-02-18)

Author: Ericson2314